Poly(hydroxyalkanoic acid) (PHA) such as polylactic acid (PLA) is a resin comprising renewable monomer such as production by bacterial fermentation processes or isolated from plant matter that include corn or sweet potatoes. The resin can be used for thermoformed packaging articles such as cups, trays, and clam shells. Generally, the resin is first extruded into an amorphous sheet and formed at about 100° C. into finished articles. The un-oriented sections of the articles do not fully crystallize because PLA crystallizes too slowly in high speed thermoforming equipment. As PLA grades popular for thermoforming have a Tg of 55° C., the thermoformed article may start to crystallize during storage if the temperature is near and above 55° C. Crystallization causes the articles to wrap and self-stick. Protection of PLA articles from 55° C. is not practical because shipping trucks experience 65° C. or higher. Furthermore if PLA is to be useful with hot-fill foods, it must resist deformation at about 80° C.
One may anneal articles such as cups in the molds but doing so may greatly increase the cost for making the articles. A preferred method is to increase the crystallinity or rate of crystallization by use of a nucleator for PLA. Many nucleators exist such as talc, sodium salt of saccharin, calcium silicate, sodium benzoate, calcium titanate, boron nitride, copper phthalocyanine, isotactic polypropylene, low molecular weight poly(lactide) and polybutylene terephthalate, but are not used because the nucleators introduce haze or opacity to the otherwise transparent PLA articles or organoleptics thereby impairing the value of the articles. See, e.g., U.S. Pat. Nos. 6,114,495, 6,417,294, and WO 03014224.
Therefore, there is a need to crystallize PLA during thermoforming at a reasonably fast rate without introducing haze or other objectionable properties to enable one to process PLA into articles wherein the un-oriented portion of the article crystallizes at such fast rate thereby providing heat resistance above the glass transition temperature of the PHA.